1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, which employs an electrophotographic printing process or the like.
2. Related Background Art
Up to now, for example, in an image forming apparatus for forming a monochromatic image or a color image by employing an electrophotographic printing method, an unfixed toner image is formed on a recording material by employing a transfer method or a direct method. In the image forming apparatus, the toner image is fixed onto the recording material by heat and pressure to obtain a permanent fixed image. As a fixing means currently mostly adopted, there is given a fixing device which employs a heating roller method in view of high speed, safety, and the like.
The heating roller method is a method in which a fixing roller heated by a heating member such as a halogen heater and a pressure roller provided so as to oppose to the fixing roller are brought into pressure contact with each other to form a mutual press-contacting portion called a nip portion, and the recording material bearing the unfixed toner image is allowed to pass between both the rollers to be heated.
The fixing roller made of a metal such as iron or aluminum is mainly used, so a heat capacity thereof is large. Accordingly, for example, a long ramp-up period from a few minutes to ten and several minutes is required to raise the temperature of the fixing roller up to around 190° C., which is an applicable temperature.
In an apparatus such as a copying machine, power is supplied to the fixing roller and the temperature thereof is maintained to a preheating temperature, which is a little lower than the applicable temperature even at a stand-by time where a user does not perform printing. When printing is performed, the fixing roller is immediately heated up to the applicable temperature, thereby being capable of reducing waiting time of the user until the temperature of the fixing roller rises.
In recent years, a growing awareness of environmental conservation has led to establishment of regulations in view of energy saving in a plurality of countries. It is preferable that a power supply is reduced to zero when apparatuses are not used, since it is effective in power saving to reduce an energy consumption during a stand-by time which accounts for a large part of power consumption.
When standby power is reduced to zero in a conventional structure, it takes a long time to heat up the fixing roller when the fixing roller is used again, thereby making it less convenient for the user. Therefore, there is required a structure with which the temperature of the fixing roller can immediately be raised.
In order to shorten the temperature rising time of the fixing roller, it is preferable to increase input energy per unit of time, that is, normal rated power. For example, in an apparatus capable of printing with high speed, power supply voltage is set to 200 V. However, in common offices in Japan, power of 100 V-15 A is commonly used, and an upper limit of the power is 1,500 W. Thus, an extra building construction work relating to the power is required for an installation location in order to correspond to the power of 200 V. Such the solution is not general.
An image forming apparatus described in JP 10-282821 A is provided with an auxiliary power supply as a power supply for operating fixing means in addition to a main power supply. In particular, JP 10-282821 A discloses use of a secondary cell as an electrifiable auxiliary power supply. To be specific, in such the apparatus, the secondary cell is charged during a standby state of the apparatus, and the secondary cell is used in addition to the main power supply at a warming-up time of the apparatus, thereby making it possible to supply a large amount of power. A representative example of the secondary cell is a lead acid battery.
However, since a conventional image forming apparatus including an auxiliary power supply supplies power only to a fixing device, effective utilization of power of the auxiliary power supply has been insufficient.